The Mona Lisa, by Leonardo da Vinci, is possibly the most well-known piece of art in the world. One of the reasons why people wax rhapsodic over the painting is the tiny hint of a smile on the woman's face. That fascinating expression has launched a thousand silly theories. Find out about the physics, the biology, the sociology, and the conspiracy behind the Mona Lisa's smile.

10. Because of Your Stupid Eyes

The human visual system, it seems, is not designed to handle much. The central area of the eye is meant to handle colors and details, while the edges see blobs, shadows, and black and white. When you focus on the eyes of the Mona Lisa, which people tend to do since we're socialized to look in each other's eyes, the outer region of the eye enhances the shadows of the mouth, enhancing the smile. When you look back at the mouth, the smile dims. The prominent cheekbones and deep shadows help this effect, bringing it out in the Mona Lisa rather than any other painting.

9. Because She Ate Fatty Foods

One doctor says the build-up of fat in the cheeks that cause the shadows to be thrown around the mouth and the corners of the mouth to turn up is a disorder caused by high cholesterol. She had xanthelasma, the accumulation of cholesterol just under the skin that made her cheeks pooch out and pull her mouth with it.

8. Because She Was Knocked Up

The most popular candidate for the Mona Lisa was Lisa Gherardini, the wife of Florentine wine merchant Francesco del Giocondo. Her position would increase, and perhaps her happiness as well, if she bore him babies. High-resolution scans taken in 2004 were precise enough to reveal the different levels of the strokes of paint, and details such as embroidery that age had scrubbed from the painting. They also hinted at a veil, light and gauzy, wrapped around her shoulders. This was what women wore when they were pregnant or nursing. It's possible that she was smiling about her soon-to-be bundle of joy.

7. Because Random Photons are Flying Through the Air

Scientists showed images of the Mona Lisa with different amounts of 'static' added on top of the image. Dark flecks that clustered around the corners of the mouth and lifted it made her seem happier. Bright flecks that seemed to flatten her mouth made her seem more somber. This effect is called visual noise, and it follows us around throughout our lives. For the most part we ignore it, because we stay in well-lit rooms and only see images for a moment before they change. But the Mona Lisa is carefully placed in a dimmer room. It is a still image, which most people aren't used to staring at for long. It's also painted in a deliberately smoky style with lots of blending shadows. A random photon that hits our eyes could light up one part of the painting's face for a moment, while a suddenly and wrongly activated color-receptor might darken another part of her face for a split second. The result is a changing, ambiguous expression that keeps us baffled. Even physics is against us when we look at this painting.

6. Because She Had Bell's Palsy

Bell's Palsy has been one of the theories put forward about the mysterious smile of the Mona Lisa. Named by Charles Bell in the early 1800s, its cause is not entirely determined, but it is thought to be the result of a persistent and otherwise asymptomatic infection. It causes the nerves in the face to become damaged and the face to become paralyzed in a smile.

5. Because She Had Missing Teeth

Some say there is a scar on the Mona Lisa's upper lip that indicates that someone or something hit her in the face. This knocked out her front teeth. Her front upper lip dropped into the space where the missing teeth were, and the corners turned up.

4. Because You've Been Staring at a Dark Screen

There are many kinds of cells in the eyes. One kind of cells only reports in when their centers are lit up - allowing people to see pinpoints of light. Another kind only reports in to the brain when the center is dark - allowing people to see dark shapes on bright backgrounds. People who have been staring at dark screens for a long time are much more able to see a smile on the Mona Lisa than people who have been staring at white screens. The dark cells are over-used, and the bright cells are fresh, and the combination of exhaustion of darkness and readiness of light lets people see the bright areas of the painting and get the impression of a smile.

3. Because Leonardo Loved His Mama

The Mona Lisa was one of the few paintings that we know was revered in its time, by its own creator. Leonardo was not the one to name it - he never gave the painting a title - but it is established that he brought the painting with him wherever he went. At a time when the only way to get anywhere was in a leaky boat or a horse-drawn cart, that meant he really liked it. No less an analyst than the great Sigmund Freud suggested that Leonardo loved it, and gave it that soft expression, because he had painted a repressed impression of his mother. It would explain a few things, including why scans showed that maternity shawl that wasn't visible to the eye. Maybe Leonardo added it in and then obscured it, just so he alone would know it was there. She smiles because she is Leonardo's best memories of his own mother.

2. Because She Was Smiling At Her Lover

Some people think the smile is less maternal and more come-hither. There are two candidates for the Leonardo's lover theory. One is a poor girl turned courtesan turned duchess. Her name was Bianca Giovanna Sforza, her nickname was 'The Tigress,' and she once did time for trying to poison a pope. This painting would have been done when she was in her forties, and well-established as a noblewoman - but she was known to take lovers and a known genius such as Da Vinci would not have been a bad catch.

The other candidate is a youth who may have inspired not only the Mona Lisa but many Da Vinci paintings of St John the Baptist and one called "The Angel Incarnate." His name was Gian Giacomo Caprotti. He was an apprentice and then a companion and researcher of the artist. He was nicknamed Salai, and one modern art historian claims that close scans of the Mona Lisa's eyes reveal an 'L' for Leonardo and an 'S' for Salai. Others say the letters are tricks of the cracks in the painting, or just aren't there at all. Either model might very well be an explanation for why the painter took this painting with him everywhere.

1. Because She is Da Vinci Himself Grinning At Us Through Time

Over time, people have noticed some similarities between the Mona Lisa's facial size and the pattern of her features, and the features of the artist. Da Vinci would have painted it somewhere between forty and fifty, and he had a full beard in his other self-portraits, so there would have been a lot of smoothing and smudging. Still, the shape of the eyes and the placement of the mouth are extremely similar. The idea is credible enough that in 2010, a team wanted to exhume his body, scan his skull, and compare its size and shape to the Mona Lisa to be sure. Not much has been heard since then, which indicates that the idea doesn't carry that much weight, but still, the smile could be Leonardo revealing his skill in more ways than one.